The improved storage dielectric of the present invention is especially useful in direct viewing bistable storage tubes which are employed in storage type cathode ray oscilloscopes, but may also be used in radar, and sonar apparatus as well as in other display devices such as those employed as part of data transmission systems. Previous bistable storage tubes employing phosphor storage dielectrics have had a slow "writing speed" which is the maximum velocity of deflection of a writing beam which will still produce a stored charge image, due to the relatively low secondary electron emission of the phosphor material. In order to increase this writing speed it has previously been proposed to add higher secondary emissive material, such as magnesium oxide, to the phosphor storage dielectric by loosely mixing the particles of magnesium oxide with particles of phosphor material. While this does increase writing speed, unfortunately it also has the disadvantage of greatly reducing the brightness of the light image emitted by the phosphor material due to the large amount of magnesium oxide required. As a result previous bistable storage tubes employing phosphor storage dielectrics either have a high brightness of about 6 foot-lamberts but relatively slow writing speed of approximately 25,000 centimeters per second obtained by omitting any secondary emissive additive, or have a relatively low brightness of about 2 foot-lamberts and a high writing speed of 100,000 centimeters per second by using such secondary emissive additive. Thus previously it was thought that both high brightness and fast writing speed were not possible when using a storage dielectric of phosphor material.
While the so-called transmission type or grid control storage tubes having a mesh storage target with a non-phosphor storage dielectric and a separate phosphor viewing screen are capable of fast writing rate and high brightness, these are much more complicated, expensive and delicate tube structures than the simplified storage tube of the present invention in which the storage dielectric is itself the phosphor material which emits a light image, so that such phosphor storage dielectric may be provided as a layer on the glass face plate of the tube envelope.
The improved storage dielectric of the present invention solves the above-mentioned problem by providing the high secondary emissive material as a coating of smaller particles on the surface of the phosphor particles so that such secondary emissive material is used most efficiently thereby greatly reducing the amount of secondary emissive additive needed to increase the writing speed to the desired speed. By using a lesser amount of high secondary emissive material, the storage dielectric of the present invention produces a light image of higher brightness due in part to the greater percentage of phosphor material, and in part to less attenuation of such light image by the secondary emissive material. In addition, by positioning the secondary emissive material only over a portion of the surface of each phosphor particle it enables the low velocity flood electrons to strike both the phosphor particle and its secondary emissive coating simultaneously which makes more efficient use of the phosphor material since the secondary emissive material does not prevent the electrons from striking the phosphor. It should be noted that the low velocity flood electrons are only accelerated through a potential difference of approximately 200 to 500 volts, so that they will not penetrate through a layer of secondary emissive material to the phosphor particle in the event that such phosphor particle is completely masked from the electrons by the high secondary emissive material which occurs in the prior storage dielectric employing a mixture of phosphor material and secondary emissive material.
The method of manufacture of the storage dielectric of the present invention forms the high secondary emissive material from a substance coated on the phosphor particles, so that the secondary emissive is intimately bonded to the phosphor particles as it is formed. However, in the previous storage dielectrics the secondary emissive material is formed separately before it is mixed with the phosphor particles, and such secondary emissive material is not bonded to the phosphor particles.
The phosphor storage dielectric of the present invention can be made porous to enable the collection of secondary electrons through such storage dielectric and the addition of secondary emissive material does not interfere with such collection because it is bonded to the phosphor particles and does not fill the pores between adjacent phosphor particles, unlike previous storage dielectrics in which it is merely mixed with such phosphor. In addition, it has been found that the storage dielectric of the present invention has a longer useful lifetime than previous phosphor storage dielectrics.
It is therefore one object of the present invention to provide an improved bistable storage apparatus which is capable of fast writing speed in producing the stored charge image and emits a light image of high brightness corresponding to the stored charge image.
Another object of the present invention is to provide an improved storage tube of simple and economical construction in which a storage dielectric of phosphor particles and high secondary emissive material coated on such phosphor particles is employed.
Still another object of the present invention is to provide an improved storage dielectric of long, useful lifetime which is capable of bistable storage of a charge image produced thereon at a fast writing speed and which emits a light image of high brightness corresponding to the stored charge image by employing phosphor particles coated with high secondary emissive material bonded to such phosphor particles.
A further object of the present invention is to provide an improved method of manufacture of a phosphor storage dielectric capable of bistable charge image storage which is simple and reliable, in which a high secondary emissive material is formed from a substance previously coated on the phosphor particles so that the secondary emissive material formed is simultaneously bonded to the phosphor particles.
An additional object of the present invention is to provide an improved storage dielectric of phosphor particles coated by smaller particles of high secondary emissive material bonded to the phosphor particles to enable a small amount of secondary emissive material to be used efficiently to increase the writing speed of the storage dielectric without greatly decreasing the brightness of the light image emitted by the phosphor particles and, at the same time, providing a longer useful lifetime for the storage dielectric.